Focusing on certain spectral lines as activity indicators sensitive to chromospheric (e.g., H\(\alpha \) , Ca ii infrared triplet) or photospheric (e.g., TiO) effects on active M dwarfs can oftentimes be successful in determining the star's rotational period [1]}. Even then, there is no universal approach because it is still not unique as to which activity indices do peak at the rotational period and under what conditions, as pointed out by [2]}.
Moreover, efforts for identifying which spectral lines, in general (i.e., not focusing on already-known specific lines), seem to be more activity-sensitive than others have been fruitful for a selection of G–K dwarfs [3]}, [4]}, [5]}, [6]}, [7]}, [8]}. Such an approach proves to be rather challenging for M dwarfs, where the spectra contain a forest of blended lines, making it almost impossible to find the continuum [9]}, [10]}, [11]}, [12]}. This approach, however, seems to have been successful for stars that show a clear stellar activity impact on the RVs (e.g., EV Lac, Lafarga priv. comm.).
